Photomultiplier tubes are radiation detectors employed in diverse applications including spectroscopy, astronomy, biotechnology, remote sensing, medical imaging, nuclear physics, and laser ranging and detection. Photomultiplier tubes exhibit excellent sensitivity, high gain, and low-noise characteristics, and further, photomultiplier tubes with relatively large photosensitive areas are feasible.
A photomultiplier tube is a vacuum tube device that is commonly comprised of a radiation-sensitive photocathode that emits secondary electrons in response to photons incident on the photocathode, various dynodes which create an electron cascade from the secondary electrons emitted by the photocathode, and an anode in which a current is induced in response to the electron cascade effected by the dynodes. The anode current is sensed in external circuitry as an indicator of the radiation impinging on the photocathode. The photocathode, dynodes, anode, and other electrodes are sealed in a vacuum enclosure. The vacuum tube has a transparent faceplate window to admit radiation that impinges on the photocathode. Variations on photomultiplier tube design include the use of focusing electrodes, multiple anodes, microchannel plates and the like. Image tubes and image intensifiers work on similar principles as photomultiplier tubes, and thus can be included in applications of the present invention.
An external high-voltage power supply and voltage divider network are used to appropriately voltage bias the electrodes. In order to detect radiation with high gain and linear response, the photocathode, dynodes, anode and other electrodes, grids, or plates of the photomultiplier tube must be voltage biased with the proper polarity and voltage levels. The present invention is, in fact, predicated on modifying the response of the photomultiplier tube by modulating voltage bias of one or more electrodes of the photomultiplier tube.
Two representative types of photomultiplier tubes will be briefly described in order to facilitate discussion of the invention. FIG. 1 shows a cross-section of a photomultiplier tube comprised of several electrodes enclosed in an evacuated tube 102 sealed at one end with a stem plate 104, and at the other end with a transparent glass faceplate 106. A photocathode 108 is formed as a coating of photoemissive material on the inside of the faceplate. A focusing electrode 110, several dynodes 112, 114, 116, 118 and an anode 120 are situated in the enclosure. Various particular electrode shapes and arrangements are possible and common, however, the present invention is not limited to a specific type of photomultiplier and will find application to virtually any gateable high-voltage device.
The electrodes can be biased by independent voltage supplies 122 as shown. In practice, the electrodes are normally biased by a single high-voltage power supply that sources a voltage divider network that in turn produces a succession of electrode biasing voltages. An aspect of the invention is to utilize this voltage divider network both for the gating circuitry and for the generation of the gating voltage pulse, circumventing the need for additional high-voltage power supplies.
Photons 124 incident upon the photocathode cause the emission of electrons 126 which impact dynode 112, causing secondary emission of more electrons 128. The process is repeated among the several electrodes creating a cascade current of secondary electrons that increase in number as the cascade proceeds from the photocathode to the anode. Upon impact with the anode 120, a current is induced in the anode which develops a voltage across a load resistor 130. This voltage is indicative of the radiation incident on the photocathode that initiated the secondary electron cascade. In normal operation of the photomultiplier tube, the electrode polarities are such that electric fields are created between adjacent electrodes to accelerate electrons and direct their impact on the appropriate adjacent electrode. An optional focusing electrode 110 is sometimes included to collimate electrons emitted by the photocathode and focus those electrons on dynode 112. If any one of the electrode voltage bias polarities is reversed, the secondary electron cascade will be frustrated, as indicated, for example, by the path of secondary electron 132 which is repelled by a reverse-bias between the photocathode and focusing electrode. This effect can be used to great diminish the anode current caused by photoemission from the photocathode. Such modification and control of the secondary electron emission current by way of altering the electrode bias voltage polarity is most effective when applied to the photocathode, focusing electrode, or one of the nearby dynodes that figure in the initiation or early stages of the secondary electron cascade.
FIG. 2 shows another prevalent type of photomultiplier, similar to that of FIG. 1, except that the several dynodes are replaced by microchannel plates. As is ommon to essentially all photomultiplier devices, the electrodes and/or plates are arranged in an evacuated tube 202 sealed at one end with a stemplate 204, and at the other end with a transparent glass faceplate 206. This example shows that the photocathode can also be realized as a separate electrode 208, rather than as a coating of photoemissive material on the transparent faceplate as indicated in FIG.1. As in the previous example, the electron cascade initiated by photoemission of electrons 210 in response to radiation 212 incident on the photocathode induces a current in anode 214 which develops a voltage across a load 216 that is representative of the radiation incident on the photocathode. Microchannel plate(s) are generally comprised of a thin sheet of lead glass in which an array microscopic channels have been etched through the sheet extending from one face of the sheet to the opposite face. The channels have diameters that can range from 10 to 100 microns. Each channel functions as a continuous dynode structure. The faces of the microchannel sheet are coated with metal that provide electrical contact and permit a bias voltage of several hundred to a several thousand volts to be imposed across the thickness of the sheet. The example of FIG. 2 shows two microchannel plates 218 and 220, but other versions of this type of device may have a single microchannel plate or several microchannel plates. The electrodes are voltage biased—here indicated by separate voltage sources 218. Also as before, in practice the several electrode voltage bias levels are produced by a voltage divider network and a single high-voltage source. The voltage biasing requirements for this type of photomultiplier tube are somewhat simpler than that of FIG. 1 since there are significantly fewer electrodes due to a microchannel plate replacing a number of dynodes.
In many applications, the high sensitivity and limited operating range of a photomultiplier tube necessitates control of the photomultiplier tube responsivity. Accordingly, the ability to switch the photomultiplier tube between an ON and OFF state is referred to as “gating” and is generally useful—and often critical—in such applications. In the ON state, the photomultiplier tube generates an appreciable anode current in response to the absorption of photons in the photocathode. In the OFF state, the photomultiplier tube is non-responsive, in that the anode current is relatively small—if not negligible—regardless of whether radiation is impinging on the photocathode. Thus, the photomultiplier tube can be controlled by a gating signal in that photomultiplier tube can be desensitized to radiation incident on the photocathode that would otherwise stimulate a secondary electron cascade and induce a proportionate anode current response. This gating function has considerable utility in spectroscopy and laser ranging, to mention a few of its applications.
For example, in phosphorescence and fluorescence spectroscopy, it is necessary to detect weak optical emission that follows relatively strong optical stimulation of the sample. When the photomultiplier tube is exposed to the strong excitation radiation used to stimulate the sample, persistent anode currents, dynode voltage depletions, and gain saturation effects interfere with the subsequent detection of the weak phosphorescence or fluorescence. To avoid these effects, the photomultiplier can be switched OFF during the excitation pulse, and switched ON to a high-sensitivity, high-gain state to detect the time-delayed weak emission that follows the excitation. The required switching time is typically in the nanosecond to microsecond range.
In Light Detection And Ranging (LIDAR) systems, a laser pulse is directed at a target, the reflection from which is detected by a photomultiplier tube. The round-trip time of the laser pulse is an indicator of the range of a target such as, for example, a satellite, missile, or aircraft. During some stages of the laser pulse travel, there is considerable scatter and back reflection from the atmosphere. It is advantageous to switch the photomultiplier tube detector to an OFF state during this period and limit the ON state to predetermined detection “window” time period that includes the anticipated time of arrival of the laser pulse reflected from the target of interest.
Another purpose of photomultiplier tube gating is to reduce the deleterious effects of intense radiation on photomultiplier tube life. High light levels can produce sputtering of the photocathode material that can permanently damage the photomultiplier tube. This sputtering effect can be suppressed if the photomultiplier tube is gated OFF to reverse-bias the photocathode during periods of spurious or damaging high radiation intensities.
Analogous photomultiplier tube switching could conceivably be realized by some type of mechanical or optical shuttering. However, the switching speeds of conventional semiconductor opto-couplers, liquid crystals, mechanical shutters or choppers, and the like are generally too slow or of insufficient contrast for most detector applications.
Significant constraints and demands on the design of photomultiplier tube gating circuits are imposed by the combined requirements and/or specifications relating to the applied electrode voltage bias levels needed to adequately modulate response, switching speed, current draw, and power consumption. Particularly, the need to apply a relatively high amplitude voltage pulse—typically on the order of ten to 100 volts—in order to sufficiently bias an electrode to suppress or enhance the secondary electron cascade between electrodes, complicates the simultaneous attainment of both fast switching speeds and low power consumption. In fact, these two design objectives are generally conflicting, and a trade-off between high speed and power efficiency is inevitable, necessitating some design and performance compromises. However, improved circuit designs can make this trade-off more favorable. Moreover, it would be convenient and less costly if the high-voltage source and associated voltage divider network used to statically bias the photomultiplier tube electrodes could also be used for generating the gate voltage and powering the associated gating circuitry. In such a case, a gate voltage pulse sourced by the voltage divider network would be applied to the appropriate electrode under the control of a supplementary gate voltage switching circuit that is also powered by the voltage divider network.
As there are a wide range of specifications for gating circuits according to the diverse applications of photomultiplier tubes, it is not surprising then that there are many variations and performance characteristics of photomultiplier tube gating schemes and supporting circuitry. The present invention adds to the stock of photomultiplier gating circuits in its description of a gating circuit that: 1. is sourced by the voltage divider network and thus requires no additional high voltage supplies, 2. provides wide latitude in adjusting the amplitude of the high-voltage electrode bias pulses used to gate an electrode, 3. draws very small currents from the photomultiplier tube power supply, and 4. is compatible with low-voltage level transistor-transistor logic signals as are common in instrumentation such as commercial pulse generators. With regard to this last point, the excitation pulse can be synchronized with a detection window determined by selectively gating the photomultiplier tube. For example, in spectroscopy or LIDAR, the laser pulse is fired by a low-voltage signal generator, the output of which can also be used, with appropriate built-in time delays, as a triggering signal for the photomultiplier tube gating circuit. This capability can be used to limit detection intervals to the anticipated arrival times of the radiation of interest, and block the detection of radiation that falls outside this detection window. Moreover, the photomultiplier tube gain—determined partly by the electrode voltage biases—can be optimally set for sufficiently high sensitivity and responsivity, without the deleterious and interfering after-effects of any intense or spurious radiation incident upon the photocathode at times immediately preceding the detection interval.